N-way coaxial signal injector with axial feeds

A method of combining a plurality of input signals in an axially-fed RF power combiner, the method comprising:

• receiving the plurality of input signals through a plurality of corresponding coaxial vacuum feedthroughs arranged axially around a vacuum coaxial transmission line;
• transitioning the plurality of input signals within a vacuum envelope of the vacuum coaxial transmission line to quasi-TEM mode signals along an inner conductive surface the vacuum coaxial transmission line;
• and
• azimuthally spreading and combining the quasi-TEM mode signals within the vacuum envelope to generate a composite TEM mode signal that propagates within the vacuum envelope, the composite TEM mode signal comprising a fundamental-mode TEM output,
• wherein an increasing gap is provided between the inner conductive surface and an outer conductive surface of the vacuum coaxial transmission line to transition the plurality of input signals from each coaxial vacuum feedthrough to quasi-TEM mode signals within the vacuum envelope of the vacuum coaxial transmission line.

An axially-fed signal injector, comprising:

• an RF power combiner comprising a vacuum coaxial transmission line having a plurality of coaxial vacuum feedthroughs configured to receive a corresponding one of a plurality of input signals, the feedthroughs arranged axially around the vacuum coaxial transmission line;
• and
• a cathode housed within a hollow portion of the vacuum transmission line,
• wherein the vacuum coaxial transmission line comprises a vacuum envelope having an annular shape, the vacuum envelope provided between an inner conductive surface and an outer conductive surface, the inner conductive surface being an inner conductor of the vacuum coaxial transmission line,
• wherein the inner conductive surface of the vacuum coaxial transmission line comprises a cylindrical conductive base and a plurality of radially-aligned conductive ridges azimuthally distributed within the vacuum envelope of the vacuum coaxial transmission line, wherein a respective one of the conductive ridges is coupled to a center conductor of a corresponding one of the coaxial vacuum feedthroughs, and
• wherein the conductive ridges have a taper to provide an increasing gap between the conductive ridges and the outer conductive surface.